compilers: Update gnu_inline test to work correctly with Clang >= 10

Which has changed the way it handles gnu_inline in C++.

mesonbuild/compilers/c_function_attributes.py

@@ -127,4 +127,7 @@ CXX_FUNC_ATTRIBUTES = {
          'static int (*resolve_foo(void))(void) { return my_foo; }'
          '}'
          'int foo(void) __attribute__((ifunc("resolve_foo")));'),
+    # Clang >= 10 requires the 'extern' keyword
+    'gnu_inline':
+        'extern inline __attribute__((gnu_inline)) int foo(void) { return 0; }',
 }